The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Various chemical processes may require multiple actuators to supply an input for a main controlled parameter. FIG. 1 illustrates such a process in which two valves 14 and 26 serve as actuators that supply heat to a nitrogen (N2) pumping unit for maintaining the temperature of the N2 at a desired setpoint. In this process, pumps 10 and 22 are utilized to convey a heat transfer fluid such as water to respective valves 14 and 26 via respective streams 12 and 24. Pumps 10 and 22 may be, for example, positive displacement pumps. Valves 14 and 26 are desirably heat throttle valves, each having a high pressure side near its inlet and a low pressure side near its outlet. Thus, the temperature of the heat transfer fluid increases as it passes through each valve. The heat transfer fluid exiting valve 14 is conveyed directly to a heat exchanger 18 via stream 16, whereas the heat transfer fluid exiting valve 26 is conveyed to a hydraulic motor 30 via hydraulic line 28 before being passed on to heat exchanger 18 via stream 32. A N2 line 34 is also pumped to heat exchanger 18. Within heat exchanger 18, energy may be transferred from the heat transfer fluid to the N2, thereby causing the temperature of the N2 to increase. Further, at least a portion of the heat transfer fluid stream 20 that exits heat exchanger 18 may be recycled back to respective pumps 10 and 12 via respective recycle streams 38 and 40.
Controlling a multiple actuator process can be very difficult. Each actuator may have independent constraints that may be affected by outside criteria. For example, an independent constraint of a valve is its maximum pressure, which is a function of its position and of the viscosity of the fluid passing through the valve. In a conventional process control system, an input or driving signal may be transmitted to a first actuator until its maximum constraint is reached. At that point, a hard stop occurs in which the input is suddenly diverted to the next actuator. Unfortunately, hard stops may degrade the performance of the controller being employed in the process control system. A need therefore exists to develop a control system to control multiple actuators having independent constraints without subjecting the system to hard stops.